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Minato-ku, Japan

Yamamoto T.,Kyoto University | Sakamoto H.,Transnuclear Tokyo Ltd.
Optics Communications | Year: 2016

A frequency domain Monte Carlo method is applied to near-infrared optical tomography, where an intensity-modulated light source with a given modulation frequency is used to reconstruct optical properties. The frequency domain reconstruction technique allows for better separation between the scattering and absorption properties of inclusions, even for ill-posed inverse problems, due to cross-talk between the scattering and absorption reconstructions. The frequency domain Monte Carlo calculation for light transport in an absorbing and scattering medium has thus far been analyzed mostly for the reconstruction of optical properties in simple layered tissues. This study applies a Monte Carlo calculation algorithm, which can handle complex-valued particle weights for solving a frequency domain transport equation, to optical tomography in two-dimensional heterogeneous tissues. The Jacobian matrix that is needed to reconstruct the optical properties is obtained by a first-order "differential operator" technique, which involves less variance than the conventional "correlated sampling" technique. The numerical examples in this paper indicate that the newly proposed Monte Carlo method provides reconstructed results for the scattering and absorption coefficients that compare favorably with the results obtained from conventional deterministic or Monte Carlo methods. © 2015 Elsevier B.V.

Yamamoto T.,Kyoto University | Sakamoto H.,Transnuclear Tokyo Ltd.
Annals of Nuclear Energy | Year: 2014

A new Monte Carlo method to calculate two types of kinetics parameters, the effective delayed neutron fraction (βeff) and the prompt neutron generation time (Λ), is proposed in this paper. The new method uses perturbation techniques in which extra delayed neutrons or a fictitious 1/v-absorber is added to the unperturbed system to calculate the βeff or Λ, respectively. In the new method, the perturbation is added as a complex-valued perturbation. This paper conjectures that the change in the eigenvalue due to the perturbation is accurately approximated by the imaginary part of the eigenvalue of the complex-valued perturbed equation. The conjecture is corroborated by certain numerical tests presented in this paper. A Monte Carlo calculation algorithm is established to solve the complex-valued perturbed eigenvalue equation. One single Monte Carlo calculation to solve the complex-valued eigenvalue equation yields highly accurate approximations of the exact kinetics parameters with much less computational costs compared with the previously proposed method that uses multiple Monte Carlo runs. © 2014 Elsevier Ltd. All rights reserved.

Yamamoto T.,Kyoto University | Sakamoto H.,Transnuclear Tokyo Ltd.
Progress in Nuclear Energy | Year: 2016

This paper presents new findings in predicting the void fraction or void transit time of void-containing water flow using the results of multi-group neutron noise transport calculations. The neutron noise transport equation in the frequency domain is solved with a recently developed Monte Carlo method that uses complex-valued weights. In the calculations, the noise of thermal neutrons that penetrates a two-dimensional channel of void-containing water is obtained to predict the void properties. The thermal neutron noise is influenced by the slow down from the neutron noise in the higher energy range and by the transport of the thermal neutron noise. This influence becomes more notable in a lower void fraction or in a wider water channel. If the thickness of the channel is small enough, the void transit time can be accurately predicted using the CPSD between detector pairs aligned in the flow direction. In a wider channel, an anomalous void transit time would be obtained from the CPSD. The results of the multi-group calculation show that the APSD in a lower void fraction deviates from the Lorentzian form that holds in the one-energy group approximation. This deviation is caused by the higher energy neutron noise, whose frequency characteristics are different from the thermal neutron noise. Assuming that the one-energy group approximation is applicable, the relationship between the break frequency of the APSD and the neutron diffusion length or neutron age is clearly observed, which suggests that the measurement of the APSD could lead to the prediction of void properties. © 2016 Elsevier Ltd.

Yamamoto T.,Kyoto University | Sakamoto H.,Transnuclear Tokyo Ltd.
Annals of Nuclear Energy | Year: 2015

A Monte Carlo method that calculates the transient behavior of neutron flux in the frequency domain is proposed in this study. A time-dependent neutron transport equation, in which the neutron source term varies over time while other properties remain constant, is Fourier transformed to obtain the transport equation in the frequency domain. The complex-valued transport equation in the frequency domain is subsequently solved with the complex-valued weight Monte Carlo method for each frequency contained in the Fourier transformed source intensity. The effect of delayed neutrons can be easily included in this frequency domain transport equation. Using the inverse Fourier transformation of the neutron flux in the frequency domain, we can obtain the time variation of the flux. This method is applicable to transient analyses of a subcritical system with a time-varying neutron source intensity. Several numerical examples indicate that the newly developed frequency domain calculation method provides good results compared to the time-dependent calculation method in the time domain. The computation time in the frequency domain is significantly shorter than that in the time domain, particularly for a nearly critical system. © 2015 Elsevier Ltd. All rights reserved.

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